Tiazofurin, a new antitumor agent undergoing clinical evaluation, was found to be an effective inhibitor of nucleoside transport at concentrations achieved in patients' plasma. Inhibition is due to: (a) a direct competition for the transport protein and (b) an indirect effect on uridine/cytidine kinase mediated by an expansion of the UTP pool. 3-Deazauridine, presumably as the nucleoside triphosphate, was found to act intracellularly as a fraudulent allosteric feedback regulator of CPS-II and uridine kinase in cultured L1210 cells. Experiments are underway to determine if similar effects are achieved in vivo and then to exploit this new property of 3-deazauridine for chemotherapeutic benefit. The metabolism of VP-16 and adriamycin was studied with regard to their effects on lipid peroxidation.